1. Field of the Invention
The present invention relates to a linear motion device such as ball screw and linear guide for use in the feed mechanism for various apparatus and more particularly to a linear motion device which is used at a low preload in environments requiring high operating characteristics and durability.
2. Description of the Related Art
This type of a linear motion device includes an inner member, an outer member, and a plurality of balls which are interposed therebetween as rolling elements in such a manner that they can be endlessly circulated.
In a ball screw for example, a nut which is an outer member is fitted on the outer periphery of a screw shaft which is an inner member. The outer periphery of the screw shaft and the inner periphery of the nut each have a helical ball rolling groove formed thereon. These ball rolling grooves are formed opposed to each other to form a ball rolling path.
The nut includes a return path formed therein extending from one end of the ball rolling path to the other. The ball rolling path and the return path form a current of endless circulating path in which a plurality of balls are disposed adjacent to each other.
In such a ball screw which is a linear motion device, the rotation of the screw shaft and the nut relative to each other causes the balls to be endlessly circulated in the endless circulating path while rolling therein, allowing the nut to make linear movement relative to the screw shaft.
In a linear guide, a slider which is an outer member is fitted on the exterior of a guide rail which is an inner member. A ball rolling groove is formed along the longitudinal direction of the guide rail on the opposing side of the guide rail and the slider. These ball rolling grooves are opposed to each other to form a ball rolling path.
The slider includes a return path formed therein extending from one end of the ball rolling path to the other. The ball rolling path and the return path form a current of endless circulating path in which a plurality of balls are disposed adjacent to each other. The linear movement of the guide rail and the slider relative to each other causes the balls to be endlessly circulated in the endless circulating path while rolling therein.
During the operation of such a linear motion device, load is applied to the balls when they are rolling over the part of the ball rolling path formed by the ball rolling grooves.
Unlike ball bearings, linear motion devices such as ball screw and linear guide have no retainers for retaining the balls. In the linear motion devices, therefore, the balls undergo competition (pushing) with each other in a loaded range when they are rolling, making it likely that the operating characteristics thereof or the life of the balls and the rolling grooves can be deteriorated.
Such a linear motion device has balls incorporated as rolling elements in the endless circulating path with some clearance therebetween. Accordingly, the balls cannot come in contact with each other so far as all the balls make an ideal rolling movement. However, competition of the balls by collision is inevitable depending on the precision of the rolling grooves, the state of contact of the balls with the rolling grooves, the working conditions, etc.
When competition of the balls occurs, the balls roll with difficultly and thus undergo slippage with the rolling grooves, resulting in the deterioration of the operating characteristics of the linear motion device and the life of the balls and the rolling grooves.
In recent years, for the purpose of preventing the competition of balls with each other, it has been often practiced to provide a retaining piece interposed between the balls.
As shown in FIG. 5, a retaining piece 1 is disposed interposed between juxtaposed balls 2 to prevent the contact of the balls 2 with each other. The retaining piece 1 is received in the endless circulating path 3 of a ball screw as shown in FIG. 6 while being carried by the ball 2 at both sides thereof in such an arrangement that it can flexibly follow the angular change of the circulating path 3.
In FIG. 6, the reference numeral 4 indicates the screw shaft of a ball screw on the periphery of which a nut 5 is fitted. The outer periphery of the screw shaft 4 and the inner periphery of the nut 5 have helical ball rolling paths 7, 8 formed opposed to each other, respectively. The ball rolling groove 8 on the nut 5 and the ball rolling groove 7 on the screw shaft 4 form a ball rolling path 9.
The nut 5 includes a tube 10 provided therein to form a return path connecting one end of the ball rolling path 9 to the other. The tube 10 and the ball rolling path 9 form an endless circulating path 3 in which a number of balls 3 and retaining pieces 1 each interposed therebetween are received. In this arrangement, when the nut 5 rotates helically relative to the screw shaft 4, the balls 2 are endlessly circulated in the endless circulating path 3.
During this procedure, the retaining piece 1 makes sliding movement with the balls 2. In the case where the frictional force developed by this sliding movement doesn't prevent the rolling movement of the balls 2 on the rolling grooves 7, 8, it results in no deterioration of the operating characteristics or the life of the balls 2 and the rolling grooves 7, 8.
However, as the force applied to the retaining piece 1 by the balls 2 increases to raise the frictional force between the balls 2 and the retaining piece 1, the rolling movement of the balls 2 is adversely affected, causing sliding movement of the balls 2 with the rolling grooves 7, 8. As a result, the operating characteristics of the linear motion device or the life of the balls 2 and the rolling grooves 7, 8 is deteriorated. This phenomenon can easily occur with low preload linear motion devices showing a low surface pressure on the rolling portion or linear motion devices having a large number of balls in the circulating path 3.
Further, when the precision of the rolling grooves 7, 8 is poor or the linear motion device is under offset load due to swinging operation or the like, the force developed between the balls increases, resulting in the deterioration of the operating characteristics of the linear motion device or the life of the balls 2 and the rolling grooves 7, 8 for the same reason as mentioned above.
In order to relax these problems or attain the reduction of noise, an elastomer having elastic property may be used. However, a material having elasticity essentially has swell characteristics and thus tends to absorb an oil or water to swell.
When the retaining piece 1 swells, the clearance of the balls 2 and the retaining pieces 1 within the circulating path 3 is reduced, jamming the circulating path 3 with the balls 2 and the retaining pieces 1. Thus, a great force is developed across the balls 2 and the retaining pieces 1, adding to the frictional force therebetween. As a result, it is likely that adverse effects can be exerted on the operating characteristics and durability of the linear motion device.
It has been proposed that the retaining piece 1 has a groove-like notch 11 formed on the periphery thereof to become an elastic structure itself as shown in FIG. 7. In this case, however, the mold die for forming the retaining piece 1 is complicated or the retaining piece 1 has a complicated configuration in the outer periphery thereof, making it more likely that the retaining piece 1 can be caught by the inner wall of the circulating path 3.
In general, the retaining piece 1 is adjusted to have a slight clearance from the inner wall of the circulating path 3 and is incorporated in the circulating path 3. When the filling clearance of the retaining piece 1 is too small or the retaining pieces 1 are filled in the circulating path 3 too much, the rolling movement of the balls 2 is prevented, adversely affecting the operating characteristics or the life of the linear motion device. On the contrary, when the filling clearance of the retaining piece 1 is too great, it causes the retaining piece 1 to fall down or be caught by the inner wall of the circulating path 3. Thus, the filling clearance of the retaining piece 1 must be severely controlled. Accordingly, it takes much time and labor to adjust the filling clearance of the retaining piece